1. Field of the Invention
The present invention relates to the field of gas and oil well swabs and particularly to oil swabs which are self-actuated and use the well gas pressure to repeatedly remove quantities of accumulated liquid from the well casing.
2. Description of the Prior Art
Gas and oil well swabs having slidable cup seals and an internal passage with a valve being closed solely by the movement of a piston sliding inside a cylinder have been described in U.S. Pat. No. 4,070,134. The prior art device comprises a cylinder having a piston inserted into an open end thereof, which piston is connected through a actuator rod to the valve. The cylinder is provided with an initial charge and the piston is provided with sliding seals. The oil swab is intended to sink below a predetermined level of liquid in the casing. This causes an increase in pressure, which increase in pressure compresses the piston against the charge in the cylinder causing the valve to move against the seat. It has been found that under ideal conditions such an arrangement will function as described. However, under the existing varied operating conditions, such a device often either operates only erratically or fails altogether and remains submerged in the well.
An attempt to resolve the foregoing problems is set forth in U.S. Pat. No. 4,712,981. In that patent, the problem was identified as one in which the valve ball does not seat because a nearly seated condition causes a restriction and a pressure build-up sufficient to stop the descent of the tool but insufficient to raise the swab. Alternatively, the problem is identified as resulting from an inordinate amount of pressure being placed upon the valve, thereby jamming it against the seat and "rendering it useless." The solutions disclosed have not successfully eliminated the inconsistent operation and failure of the prior art swabbing devices.
The foregoing swabs also employ a speed brake device consisting of a free-floating steel ball which is moved, by rushing gas, upward into a restrictive opening. The turbulence of the rushing gas causes the ball to bounce violently and damage itself and damages the opening of the restrictive passageway into which it eventually moves.
In the prior art, a catching mechanism is also used to hold the swab at the top of the oil well casing, which mechanism employs a sharp nosed plunger which becomes rapidly worn due to repeated use. Such undue wear can make the catching mechanism ineffective.
It has been difficult with prior art automatic swabbing devices to determine when the swabbing device was at the top of the well and, therefore, in a position for being caught. It is important to determine when the swabbing device reaches the top of the well without removing the well head. Built-up pressure must be relieved before removing the automatic swab from the well casing.